236 ESSAY 



sufficient to occasion a difference in their quan- 

 tities of moisture. Another night, I laid on 

 the raised board a piece of pasteboard covered 

 with white paper, and close to this a second 

 piece similar to the former in every respect, 

 except that it was covered with paper black- 

 ened with ink. At daylight, I saw hoarfrost 

 upon both pieces ; but the black seemed to have 

 a greater quantity than the white. A doubt, 

 however, afterwards arose upon the accuracy 

 of this experiment likewise ; for, as the light 

 was faint, when I viewed the two surfaces, the 

 quantity of hoarfrost, though equal on both, 

 might have appeared greater on the black, than 

 on the white, from the contrast of its colour 

 with that of the former surface. But trials of 

 this kind, as Mr. Leslie* has observed, never 

 afford firm conclusions ; since a black body 

 must always differ from a white in one or more 

 chemical properties, and this difference may of 

 itself be competent to produce a diversity in 

 their powers to radiate heat. 



With the view to render the subject less com- 

 plicated, I have hitherto treated of dew, as if it 



* On Heat, p. 95. 



